Abstract

Due to urbanization, industrialization and population increase, a substantial increase occurred in the number of vehicles and hence large numbers of end-of-use tires are being disposed every year. The vast majority of these tires are stockpiled or used as a fuel for combustion which, in both cases, affects the environment detrimentally. The use of tire rubber in cement-stabilized aggregate mixtures (CSAMs) will ensure beneficial use of large quantities of these waste materials, saving natural resources and may enhance the properties of CSAMs especially these related to brittleness and sensitivity to fatigue failure.

Research was undertaken to investigate, at macro and mesoscale levels, the effect of both rubber and degree of stabilization and their combination on the behaviour of CSAMs in terms of the most influential pavement design properties under different static and dynamic modes of loading. These properties are strength, stiffness and fatigue. A range of testing equipment, methodologies and tools was developed, suggested and implemented to perform this investigation. Further investigation was also conducted to provide better understanding of the damage and failure mechanism through quantitative studying of the fractured surface, internal structure and surface cracking patterns under different testing modes.

The results of this study revealed that the addition of rubber has a negative effect on the compaction efficiency, compressive, flexural and tensile strengths while the stiffness, under different testing modes, was slightly reduced. In addition, a tougher mixture was produced after rubber-modification which means a change from a brittle to a more ductile behaviour. This behaviour was observed through different stiffness modulus evaluation methods. On the other hand, increase in cementation level has resulted in an increase in both strength and stiffness for both reference and rubberized mixtures. However, the decrease in the mixtures’ strength due to rubberization was more obvious in highly cemented mixtures than the lightly cemented ones. On the other hand, a greater decline in the mixtures’ stiffness, due to rubber incorporation, was observed at low cement contents. This behaviour is related to the void-like behaviour which depends, to large extent, on the relative stiffness between rubber and surrounding matrix.

Quantification of the fractured surfaces and cracking pattern utilizing the photogrammatry and fractal dimension concepts, respectively, revealed that the addition of rubber resulted in rougher and more tortuous cracks and increases disperse-ability of these cracks. This means the rubber-modification changed the cracking pattern which implies better load transfer through the cracks and less risk of reflection cracking.

The investigation of the internal structure, at mesoscale level, showed that the cracks were propagated through the rubber particles at all investigated cementation levels. This contributed to a lengthening of the crack path and to the delaying of crack propagation by absorbing and relieving the stresses at the crack tip, especially at the microcrack level. The latter mechanisms are behind toughness and fatigue improvement. Evaluation of rubber distribution revealed uniform distribution and this decrease as rubber content increases.

The results also indicated an improvement in the fatigue life for all rubber replacement levels. This was valid at all cementation levels. In terms of modulus degradability, rubberization of the cemented mixture has only a slight effect on this property while larger permanent deformation was accumulated after rubber inclusion. It was observed that the poorly cemented mixtures showed greater stiffness modulus degradation. Pavement analysis and design study showed that the decrease in the mixtures’ strength overshadowed any improvement due to both mitigation of mixtures’ stiffnesses or fatigue life enhancement. However, this is not the case for poor rubber mixtures where this mixture showed better behaviour than the reference mixtures.